The present invention relates to pads in which a gel is present within a flexible resilient casing and to a process for their preparation. Such pads are particularly useful as supports for a reclining or sitting person, in mattresses, mattress inserts, wheelchair cushions and car seats.
Gel pads useful as support for a person in a reclining or sitting position are known. A pad with a flexible, elastic nucleus composed of a PVC gel is described, for example, in U.S. Pat. No. 3,737,930. However, such PVC gels having adequate breaking strength can be formulated only with relatively high hardness. The contact pressure in the case of persons lying or sitting on such pads can not therefore be evenly distributed. Such uneven distribution of weight does not prevent development of decubitus by the user. Another disadvantage of gels of this type is the high content of softeners which must be used to obtain a comfortable pad. These softeners can migrate to the surface of the gel from which they may be removed by the person lying or sitting on the pads. Residual monomers of PVC, which are known to be toxicologically unacceptable, may also rise to the gel surface and be removed therefrom by the user of the pad.
Pads for avoiding decubitus are also described in British Pat. No. 1,541,071. The pads disclosed therein contain a thixotropic gel composed of aqueous solutions of 3%-7% of magnesium silicate. However, these gels have so little dimensional stability that they can be used only within a firm frame. Such frames, however, frequently cause pressure points thereby increasing the likelihood of developing decubitus.
U.S. Pat. No. 3,663,973 discloses a pad composed of a polyorganosiloxane gel which is described as being useful for preventing pressure necroses in mammals. However, these polyorganosiloxanes are relatively expensive materials and pads based on these materials can be manufactured only at considerable cost.
The use of water-containing and/or alcohol-containing polyurethane/urea gels as pad elements or shock-preventing components is described in German Auslegeschrift No. 2,347,299. However, the gels disclosed in this Auslegeschrift contain considerable quantities of water and/or low-molecular weight alcohols. These substances are volatile, which leads to drying out and hardening of these gels over relatively long periods of storage. This hardening also occurs if the gels are encased in an elastic film because the permeability of known elastic films to gas and water vapor is not low enough to prevent the escape of the volatile components. The gels described in German Auslegeschrift No. 2,347,299 can therefore only be stored, as pad elements, for a limited time.
Another process for the preparation of pad materials is described in Japanese Patent Application No. 55-3404. These pad materials are prepared by reacting an NCO-prepolymer with a stoichiometric excess of water. However, the pad materials prepared according to this teaching also release water fairly rapidly which results in drying out and loss of the gel's starting properties. This drying out cannot be completely prevented by a film cover because the pad materials disclosed also have a limited lifetime. The high specific heat of these materials (due to the proportionately high quantity of water) is also disadvantageous. The body heat of the user is very rapidly conducted away upon contact with such pad materials resulting in an unpleasant and medically undesired feeling of coldness.
A further disadvantage of the known gel pads for avoiding decubitus is that their properties are relatively invariable. It is therefore difficult to adapt their spectrum of properties to a particular application.